The present disclosure generally relates to Passive Optical Network (PON) technology, and more particularly to a maintenance method and system for passive optical networks.
In broadband access networks, which are growing in scale, most existing local access networks (LANs) run at 100 Megabits per second (Mbps) and many large commercial companies are transitioning to Gigabit Ethernet (GE). However, in the metropolitan area core networks and the edge networks, the bandwidth capacity of synchronous optical network (SONET), synchronous digital hierarchy (SDH), and GE is abundant, which results in a bandwidth bottleneck at the access network. Compared to cable transmissions, optical fiber transmissions have many advantages, such as a large capacity, limited loss, and a strong ability to resist electromagnetic interference. Thus, with the cost of optical fiber transmission decreasing, the tendency is towards using fiber in the access network. The last mile access networks require low cost, simple structure, and ease of realization, which is a challenge to achieve technologically. Through the use of passive devices, PONs are a potential technology for broadband optical access networks.
As one of the potential technologies for broadband access networks, there are many technical advantages to PONs. First, optical access networks are the best solution for adapting to future developments, especially PONs and x-PONs that are combined with asynchronous transfer mode (ATM), Ethernet, or wavelength division multiplexing (WDM), which have proved to be a cost effective way to integrate broadband access networks. Second, by using PON technology, the entire optical distribution network is passive, small, and simple. Third, compared to cable networks, PONs can reduce the maintenance costs and avoid electromagnetic interference problems. Fourth, the PON's passive optical network unit (ONU) does not need power, so the ONU not only avoids numerous power supply problems, but also is more reliable than powered devices. Fifth, the overall network cost is lower because the passive devices share the optical fiber. Sixth, PONs support new services, especially multimedia and broadband services, and therefore PONs can strengthen the operator's core competitiveness, quicken the development of new services, benefit from new investments in optical network construction, and promote digital access networks. Seventh, to a certain degree, PONs are transparent to the transmission system and easy to upgrade.
Depending on the content that the PON carries, PON technology can be classified as ATM-based PON (APON), Ethernet-based PON (EPON), or Gigabit PON (GPON). Using APON, EPON, or GPON technology, the PON can support speeds of 155 Mbps, 622 Mbps, 1.25 Gigabits per second (Gbps), or 2.5 Gbps on the PON core fiber. By supporting multiple transmission speeds, the bandwidth assignment for each subscriber may be either static or dynamic.
According to the fiber extension, the optical access network can be classified as Fiber to the Home (FTTH), Fiber to the Building (FTTB), Fiber to the Curb (FTTCurb), Fiber to the Cabinet (FTTC), or Fiber to the Premises (FTTP), which can be referred to generically as Fiber to the X (FTTX).
One embodiment of the PON structure is shown in FIG. 1, and includes an Optical Line Terminal (OLT) located at the central office and a plurality of ONUs or optical network terminals (ONTs) located at the user locations. The primary difference between the ONU and the ONT is that the ONT is located at the customer side, while the ONU has other networks, such as an Ethernet network, between the ONU and the user. Because the differences between the ONU and the ONT are limited, the use of ONU herein refers to either the ONU or the ONT. The OLT and ONU are connected via fiber with an optical distribution network (ODN) containing passive splitters and/or couplers. In the PON, an optical fiber is laid from the service switch center to the broadband service sub-area, and then passive splitters and/or couplers separate the main fiber into several sub-channels that run to each building and service facility. The downstream direction refers to signals traveling from the OLT to the ONU, and the upstream direction refers to signals traveling from the ONU to the OLT.
FIG. 2 illustrates one embodiment of the OLT, which includes an optical module, a service processing module, a control module, and a power module. FIG. 3 illustrates one embodiment of the ONU, which includes an optical module, a service processing module, and a power module. The optical module converts optical signals received by the OLT and ONU, and includes an optical receiver and an optical transmitter. The power module is connected to the optical module in the OLT and the ONU, and provides power to the optical receiver and the optical transmitter. The power module may be controlled by a manual switch. In the OLT, the service processing module is connected to the central office (CO) upstream network interface through a central network interface (CNI). In the ONU, the service processing module is connected to the user devices through a user network interface (UNI).
Referring back to FIG. 1, in the PON the downstream data transmission process is different from the upstream data transmission process. The downstream data is broadcast from the OLT to every ONU, and each ONU uses the address information in the packet's protocol header to determine whether the destination address in the packet matches the ONU's address. Each ONU processes the packets that match its own address and ignores packets intended for other ONUs. Upstream transmission is more complex because the ODN's optical medium is shared. To avoid packet collision, the OLT control module uses time division multiple access (TDMA) to control the upstream data transmissions. Thus, specific upstream transmission timeslots are assigned to each ONU, and the timeslots are synchronized to prevent the packets from different ONUs from colliding.
In a point-to-multipoint PON, the upstream data is transmitted in TDMA mode, thus each ONU transmits time division multiplexed data to the OLT. The OLT assigns each ONU a timeslot to guarantee that only one ONU will emit light at any given time, thereby avoiding packet collisions. Under normal circumstances, the ONU's optical module is only active during the timeslot assigned by the OLT. However, a fault may occur in an optical module that causes the optical module to constantly emit light, perhaps because a malicious user set the optical module in a constant light-emitting state. If such a fault occurs in an ONU, then all of the ONUs connected to the same OLT as the faulty ONU will be deactivated.
One existing solution that may solve constant light-emitting faults is the use of an active splitter that monitors each of the splitter's sub-channels. If one of the sub-channels constantly emits light, then the OLT can disable the affected sub-channel. However, using active splitters eliminates many of the advantages of the PON by reducing the reliability of the system, increasing the amount of maintenance, and increasing the cost of the system.
Thus, a need exists for improvement in the present technology. To meet the industry demands, the improvement needs to detect and isolate the faulty ONU and avoid affecting the other, normal ONUs.